Inhibition of corrosion of phosphated metal surfaces



INHIBITION OF CORROSION F PHOSPHATE!) NETAL SURFACES William M. Le Susi, Cleveland, Ohio, assignor to The Lllllilllizfli Corporation, Wickliife, Ohio, a corporation of 0 'o r No Drawing. Application January 17, 1957 I Serial No. 634,627

11 Claims; cl. 117-127 significance in many industrial applications, and as a con sequence the inhibition of such corrosion is a matter of prime consideration in such applications. It is a particular significance to users of steel and other ferrous'alloys. The corrosion of such ferrous metal alloys is largely a matter of rust formation which in turn involves-the over all conversion of the free metal to its oxide.

The theorywhich best explains such oxidation of fertons-metal surfaces postulates'the essential presence both of water and oxygen. are'sufiicient, according to this theory, to induce the dissolution of iron therein and the formation of ferrous hydroxide until the water becomes saturated with ferrous ions. The presence of oxygen causes oxidation of the resulting ferrous hydroxide to ferric hydroxide which then settles out of solution and is ultimately converted to ferric oxide or rust.

The above sequence of reactions can be prevented, or at least in large measure inhibited, by relatively imperme able coatings which have the effect of excluding either or both moisture and oxygen from contact with the ferrous metal surfaces. Such coatings are, of course, subject to abrasion and other causes of physical deformation and to the extent that these coatings are penetrated or otherwise destroyed by such influences they are ineffective for the purpose desired. It is important that such coatings provide complete protection of all of the ferrous metal surfaces. If there is any portion of such a surface which is not so protected, regardless of how small the unprotected surface may be, the degree of protection afforded is considerably less than desirable. A satisfactory corrosion-inhibiting coating, then, must have the ability to resist substantial deformation upon impact, abrasion, etc. so -thata uniform and complete film of protection is maintained in the face of such adverse influences.

It is accordingly a principal object of this invention-to provide protection from corrosion for metal surfaces.

It is another object of this invention to provide protection-from the formation of rust on ferrous metal surfaces.

Other objects will be apparent from the ensuing description.

These objects are achieved by the process of inhibiting corrosion of phosphated metal surfaces which com-r prises applying to said surfaces a film of a productprepared by the process which comprises the reaction of (a) A zinc salt of a phosphorodithioic acid with (b) An approximately equivalent amount of an organic epoxide containing at least'4 aliphatic carbon atoms.

The zinc salt of aphosphorodithioic acid which may be used as reactant (a) in the preparation of the above described rust-inhibiting agent is easily prepared merely Even minute traces of moisture- "i. e., those prepared by the reaction of phosphorus pentasulfide with a single alcohol, either -monohydric or poly hydric.

The compositions-of this invention are derivable'fron'r phosphorodithioic acids of either-of the above types; In some particular instances those which are-prepared. from! zinc salts of simple phosphorodithioic acidsfare pref erable to those prepared from zinc salts of. mixed phosphorodithioic acids, althoughfor mostnapplic'ationsthe reaction products prepared from the-zinc-salts of mixed phosphorodithioic acids are preferable. V 1

The 'terms"simple andmixed refer, of course, tophosphorodithioic acids prepared from in the first 'case asingle-alcohol,-and 'in'the second case from a mixture of a monohydric and a polyhydric'alc'oho'l. I V

Phosphorodithioic:acids may be represented by the strueturalformula: t

in which R and R areorganic radicals attached-to theoxygen atoms through carbon atoms. The rea'ct'ion'of alcohols'with phosphorus pentasulfide yieltl's phosphorodithioi'c acids in which R and Rfare'attached to the oxygen through aliphatic carbon atoms. Correspondingly the same reaction with phenolic compounds yields phosphorodithioic acids in which'R and R are attached to these oxygens through benzenoid carbon atoms. Although it is more desirable for the purposes of this invention. to employ phosphorodithioic acids in which R and R are aliphatic this preference is predicated largely on'reasons of economy and in the absence of'suchreasons the 'aro-' matic phosphorodithioic acids would'be satisfactory'i'n many instances. Particularly useful phospho'rodithioic acids are available from such alcohols as butyl, hexyl,.. octyl, and mixtures of these' alcoholswith'ethylene glycol, propylene glycol, dipropylene' glycol and glycerol. .Par-' ticularly preferred 'phos'phorodithioic acids" may be piepared by reaction of phosphorus pent'asulfide with mixtures of isooctyl alcohol and1,2 propyleneglycol, iso octyl alcohol and dipropylene glycol, isooctyl alcohol and ethylene glycol.

For a more detailed description of the mixed phosphorodithioic acids which may be used in the preparation of the zinc salt reactants of the process of this "invention, reference maybe had to the disclosure of th'e'abo'\7eidentified 'c'ope'n'ding' application.

The epoxide reactant (b) includes, as stated previously, only those epoxides which contain within their molecular structureat least '4 aliphatic carbonatoms. It has been observed that the products obtained from the lower molecular weight epoxides, while markedly etfective in acting to prevent the corrosion of metal surfaces, arenot as effective as those products which result from 'the use of the higher molecular-weight epoxides in the hereiiidescribed process. Thusthe -lowest-moleculan eight epoxide' Which has'been' found to be effective inthe reparation of satisfactory anti-"corrosion agentsis'buteijie aide,-

The'useof this epoxide in the hereindescribed process results in the formation of products which are useful as a surface film to protect metal surfaces in corrosive environments. The use, however, ofhigher molecular weight epoxides .in the process particularly those aliphatic epoxides containing 12-20 carbon atoms per epoxy group, results in products which provide an even greater degree of protection in such environments. For this reason such epoxides as dodecene oxide, hexade'cene oxide, butyl oleate oxide, methyl oleate oxide, epoxidized soy bean oil etc. are preferred epoxides for use in this reaction.

The process conditions by which the corrosion-preventive agents of this invention maybe prepared include a temperature within the range of 20'-200 C. In most cases the reaction is exothermic so that it may insuch cases be carried out at relatively low temperatures, viz., in the range of 20-80 C. In'other cases where the reac tion'is not exothermic or where the-molecular weight of the reactants is great enough to minimize the thermal effects of an exothermic reaction it is'desirable to maintain a higher temperature, viz., in the range of'80200 C. In mostcases, however, a '--temperature within the range of 50100 C. is preferred.

The chemical reaction involved-in the process of this invention is not understood. The product which results appears not tobe asalt, but further than this'there isno information available which allows an identification of the molecular structure of the product. 'For this reason ,it is necessary to define theproduct in terms of its process of preparation. In this connection, however, it has been noted that in thosecas'es which involve an exothermic reaction, the exothermic reaction appears to involve at least about two molesof epoxide per mole of zinc phosphorodithioate. Thus one mole of a zinc phosphorodithioate will react exothermically with two moles of certain epoxides. The product of the reaction contains phosphorus, sulfur, and'zin'c in essentially the same ratio as these elements appear in the-zinc phosphorodithioate starting material.

Particular details of theprocess by which the corrosion-inhibiting agents of this invention may be prepared are illustrated in the following examples.

Example 1 it Azinc salt of a phosphorodithioic acid was prepared by the reaction of phosphoruspentasulfide with 4 moles of a mixture of equivalentamounts of isooctyl alcohol and propylene glycol, followed by neutralization of the resulting acidic product with zinc oxide. To 670 grams (1.0 equivalent) of this zinc salt there was added 'at 60-70 C. 76 grams 1.0 equivalent) of butylene oxide. The ensuing reaction was exothermic so that the indi cated temperature range was maintained by portionwise addition of the butylene oxide throughout a 30-minute period. The mixture was heated at 67-70" C. for an additional 2 hours, then flushedwith a stream of nitrogen at room temperature for 3 hours. The resulting product was a clear liquid having the following analyses:

Percent 4.6 9.2 4.6

i E xample 2 phorodithioate prepared as'in Example 1 there'was added at 70-72 C. 880 grams (2.0 equivalents) of butyl epoxy st'earate' (prepared by the epoxidation of butyl oleate).

. Although'the reaction did not appear to be exothermic the butyl epoxy steara'te' was added portion'wise over a period 'of2hours and minutes, thetemperaturebeing maintained at 70-72: C. throughout this addition and for an additional 3 hours. The productwas filtered through.

' -To '1340 grams 2.0 equivalents) of the zinc phos a siliceous filter aid to yield a clear, light yellow liquid having the following analyses:

Percent P 3 .0 S 6.0 Zn 3 .0

Example 3 The zinc salt of a phosphorodithioic acid waslprepared Percent P 3.7 S 7.7

Example 4 To 1346 grams (2.0 equivalents) of the zinc salt prepared as in Example 3 there was added at 70C. 880 grams (2.0 equivalents) of butyl epoxy stearate over a period of minutes. When the exothermic reaction had subsided the product mixture was heated for an additional 2 hours at 70-75 then filtered. The filtrate showed the following analyses: c a

. Percent P 3.6 S 7.0 Zn 3.3

Example The zinc salt of a phosphorodithioic acid wasprepared by the reaction of phosphorus pentasulfide with a 1:1 (equivalents) mixture of isooctyl alcohol and ethylene glycol, followed by reaction of the acidic'product with zinc' oxide. To a solution of 1034 grams of this salt (1.5 equivalents) in l333 grams of toluene there ,wasradded portionwise over a perioddof 3 hours 276 grams-(1.5 equivalentsyof ,dodecene oxide. The temperature was maintained at 80-90 C. throughout the addition and for one hourthereafter. The resulting product showed the following analyses:

Percent P 3.9 S 2.1 Zn 2.1

' Example 6 Percent P 11.4 S 3.6 Zn 4.0

Example 7 V To 700 grams (2.2 equivalents) of zinc di-n-butyl phosphorodithioate there was added dropwise over a.-

- period of 4 hours 330 grams (2.2 equivalents) of phenoxypropylene oxide. 60-80" C. throughoutthe addition and for-an additional The temperature was maintained at As indicated earlier the process of this invention is concerned with the application of products of the type illustrated in the above examples to a phosphated metal surface. The term phosphated refers to a treatment by which a metal surface is rendered relatively inactive to corrosion by moisture, air, etc. Such phosphating treatments are well known and are described in U. S. Patents 1,206,075; 1,247,668; 1,305,331; 1,485,025; 1,610,362; 1,980,518; and 2,001,754. They generally comprise immersing the metal surface to be treated in a hot aqueous solution of phosphoric acid and/ or phosphate salts, which solution also contains one or more inorganic salts such as zinc nitrate, calcium nitrate, copper sulfate, etc. The hot solution acts chemically on the metal surface to form an adherent crystalline deposit of bluish-grey to dark blue color. In instances of mildly corrosive environments such phosphating treatment may provide a satisfactory corrosion-proof result. For the purposes of this invention, however, a mere phosphating treatment is not sufficient. Phosphating serves to provide an initial passivation of the metal surface which must be reinforced with the coating composition described herein.

The following is presented to illustrate a practice of the invention and the advantages secured thereby: 3" x 3" phosphated steel panels, the edges of which have previously been painted with an air-drying enamel, are dipped into a solution of the zinc phosphorodithioate-organic epoxide reaction product in any suitable volatile organic solvent such as, e. g., benzene, toluene, xylene, methylethyl ketone, butyl alcohol, etc. The purpose of the enamel edge-coating is to improve the reproducibility of the corrosion test to be carried out. The panels are withdrawn from the solution at a specified rate so as to insure a uniformly thick film and the solvent is allowed to evaporate. Then the panels are placed in an enclosed cabinet which contains a 5% aqueous solution of sodium chloride at 95 C. This sodium chloride solution is agitated by means of a vigorous stream of air bubbled through the solution so as to produce a corrosive saltsteam atmosphere in the cabinet. The resistance of the panels to corrosion is measured in terms of the time required to produce a single rust spot on the panel in this atmosphere. It is apparent that the degree of protection from corrosion will be related to the thickness of the film which is deposited upon the metal surface, so that any numerical measure of protection from corrosion must take into account this factor of film thickness. Accordingly, the thickness of the film is expressed in terms of milligrams (of film) per square foot (of metal surface area). The use of this expression allows an evaluation of the eifectiveness of the corrosion-preventing films of this invention in terms of hours (of rust-free exposure in a salt-steam atmosphere), per milligrams (of film), per square foot (of metal surface area). In order to facilitate the comparison of these values for different films the salt-fog life described above has been expressed as hours/ZSO mg./ft.

The eifectiveness of the compositions of this invention as film-forming inhibitors of corrosion is shown by the following table in which each salt-fog life represents an average of results from six separate phosphated steel panels containing films of diiferent thicknesses. Each of the steel panels was first phosphated by immersion for 6 minutes in a solution containing 1.6 moles of phosphoric acid (H PO4), 0.4 mole of mono-ammonium phosphate (NH H PO4), 1 mole of zinc nitrate (Zn- (NO and 1 mole of calcium nitrate (C-a)NO at 130490 F. The panels are removed, rinsed with hot water and then with dilute chromic acid (0.5 grams/liter) at -190" F.

I Salt-tog Example Rust-proofing composition 111a Nonev 6 Product of Example 1 214 Product ol Example 2 530 Product of Example 8B Product of Example 493 Product of Example 5.- 845 Product of Example 6.. 397 Product of Example 7 265 16 A product prepared as in Example 2 except that 513 the phosphorodithioic acid is prepared from a mixture of 35 molar parts 01 isooctyl alcohol and 65 molar parts of propylene glycol.

17 A product prepared as in Example 1 except that 179 the phosphorodithioic acid is prepared as in Example 16.

18 A product prepared as in Example 5 except that 394 the glycol used in the preparation of the phosphorodithiolc acid is dlpropylene glycol.

19 A product prepared as in Example 7 except that 177 butyl epoxy steer-ate was used as the epoxide reactant.

20 A product prepared as in Example 2 except that 774 the glycol used in the preparation of the phosphoroditllioic acid was dipropylene glycol.

21 A product prepared as in Example 5 except that 924 the glycol used in the preparation of the phosphorodithioic acid was butyl epoxy stearate.

22 A product prepared by the reaction of zinc di-(2- 330 methylpentyl-4) phosphorodithloate with dodecene oxide.

23 A product prepared as in Example 7 except that 348 butylene oxide was the epoxide reactant.

Other modes of applying the principle of the invention may be employed, change being made as regards the details described, provided the features stated in any of the following claims or the equivalent of such be employed.

I therefore particularly point out and distinctly claim as my invention:

1. The process of inhibiting corrosion of phosphated metal surfaces which comprises applying to said surfaces a film of a product prepared by the process which comprises the reaction of (a) a zinc salt of a phosphorodithioic acid with (b) an approximately equivalent amount of an organic epoxide containing at least 4 aliphatic carbon atoms.

2. The process of claim 1 characterized further in that the phosphorodithioic acid of (a) is prepared by the reaction of phosphorus pentasulfide with a mixture of organic hydroxy compounds selected from the class consisting of alcohols and phenols.

3. The process of claim 1 characterized further in that the phosphorodithioic acid of (a) is prepared by the reaction of phosphorus pentasulfide with a mixture of aliphatic alcohols.

4. The process of claim 3 characterized further in that the mixture of alcohols is a mixture of a monohydric a1- cohol and a glycol.

5. The process of claim 3 characterized further in that the mixture of alcohols is a mixture of an octyl alcohol and a glycol.

6. The process of claim 3 characterized further in that the mixture of alcohols is a mixture of an octyl alcohol and dipropylene glycol.

7. The process of claim 1 characterized further in that the epoxide is an aliphatic epoxide.

8. The process of claim 1 characterized further in that the epoxide is an aliphatic epoxide containing l2-20 carbon atoms per epoxy group.

9. The process of inhibiting corrosion of phosphated metal surfaces which comprises applying to said surfaces a film of a product prepared by the process which comprises the reaction of (b) an approximately equivalent amount of an aliphatic .qepoxide containing 12-20 carbon atoms perrepoxy roups 10.--The--process of claim 9 characterized further inrthat the phosphorodithioic acid of (a) is prepared by the'reaction of phosphorus penfasulfide' with a mixture of equivalent amounts of an octyl alcohol and an aliphatic glycol m "a a V a. V a V V 11. 'I'heproeess ofwclaimfi characterized ,fufl-helijn that the epoxide of'(b) is butylepoxy stearate. l

5 1 92? ad ,y,

Salzberg et a1.

Dec. 8, i956 Ehzl3r M25 19 

1. THE PROCESS OF INHIBITING CORROSION OF PHOSPHATED METAL SURFACES WHICH COMPRISES APPLYING TO SAID SURFACES A FILM OF A PRODUCT PREPARED BY THE PROCESS WHICH COMPRISES THE REACTION OF (A) A ZINC SALT OF A PHOSPHORODITHIOIC ACID WITH (B) AN APPROXIMATELY EQUIVALENT AMOUNT OF AN ORGANIC EPOXIDE CONTAINING AT LEAST 4 ALIPHATIC CARBON ATOMS. 